A tunable laser has a wavelength of operation of the laser light (optical signal) that may be altered or tuned during operation. To continuously phase tune a laser, for example, either the length of the laser cavity (via a delay line) is kept at a fixed multiple of the emitting wavelength of the optical signal, or phase is continually shifted by an endless reset free continuous phase shifter integrated into the cavity. Keeping the cavity at a fixed multiple of the emitting wavelength is typically realized by design in mechanically tuned external cavity lasers using, for example, Littman-Metcalf geometry, and requires large opto-mechanical building blocks and extreme mechanical stability combined with control loops to maintain the desired ratio. An integrated endless reset free continuous phase shifter may be realized using a frequency shifter inside the cavity, although alternative realizations may use rotating waveplates and a lithium niobate (LiNbO3) polarization modulator. However, such modulators are difficult to operate due to dependency on temperature, intrinsic hysteretic behavior and intrinsic high losses.
An endless phase shifter or the equivalent an optical frequency shifter may be built using rotating waveplates. The waveplates may be actual waveplates that are mechanically rotating, which limits the tuning speed to unusable, low values. When building the waveplate from field induced birefringent material, problems operating this material must be solved. For example, temperature dependence and hysteresis are problematic.
What is needed, therefore, is an endless phase shifter without moving parts (e.g., mechanically rotating waveplates, that provides continuous phase tuning of a laser, for example, that is efficient and low cost.